In many technical areas of application there is a need to measure surfaces of objects and thus also the objects themselves with high accuracy. This applies in particular to the manufacturing industry, for which the measurement and checking of surfaces of workpieces are of great importance, in particular also for quality control purposes.
Coordinate measuring machines are usually used for these applications, said coordinate measuring machines enabling precise measurement of the geometry of an object surface, typically with micrometer accuracy. Objects to be measured may be, for example, engine blocks, transmissions and tools. Known coordinate measuring machines measure the surface by producing a mechanical contact and scanning the surface. Examples thereof are gantry measuring machines, as described e.g. in DE 43 25 337 or DE 43 25 347. A different system is based on the use of an articulated arm, whose measuring sensor arranged at the end of the multipartite arm can be moved along the surface. Generic articulated arms are described for example in U.S. Pat. No. 5,402,582 or EP 1 474 650.
In the prior art, a tactile sensor is used with such coordinate measuring machines as standard measuring sensor, said tactile sensor consisting of a ruby sphere, for example, which is mounted on a measuring bar. The deflection of the tactile sensor, in three mutually perpendicular directions X, Y and Z in the case of a coordinate measuring machine designed for three-dimensional measurements, is determined during the scanning by means of a switching element or distance measuring element. The location of the contact and thus the surface coordinates are calculated on the basis of the switching point or deflection distance.
In order to reconstruct the surface profile from the measurement data, it is necessary to take account of the mechanical dimensions of the sensor itself and the orientation thereof upon contact with the object surface. The sensor is embodied with a measuring tip of known geometry, typically spherical or ellipsoidal for specific applications, typically with a (main) radius of the order of magnitude of a few millimeters. In association with the present invention, the term “measuring tip” should generally be understood as (tactile) measuring sensor of any desired shape and extent, wherein said sensor need not necessarily (but can) have a tapering shape. The raw data measured by means of the coordinate measuring machine using a tactile sensor represent the measured spatial coordinates of a reference point of the measuring tip, for example of the center of the measuring tip, and are designated hereafter as relative to a “sphere center domain”. By means of a transformation algorithm taking account of the shape of the measuring tip and the orientation thereof upon contact with the object surface, the measured coordinates are usually transformed from the sphere center domain into the computationally determined object surface profile (“object profile domain”).
Owing to the physical dimensions of the measuring tip of the tactile sensor, however, the measurement resolution is restricted. The physical dimensioning of the measuring tip or the limited measurement resolution associated therewith leads to a “smoothing effect” during the measurement of rough surfaces: while elevations or peaks of an object surface can be measured almost perfectly or object-faithfully, the measuring tip of the tactile sensor, on account of its physical extent, cannot penetrate into narrow depressions of an object surface. This brings about a smoothing of the measured surface profile in a nonlinear manner by virtue of the measurement data of recessed surface regions being smoothed, while the measurement data of elevated surface regions are almost object-faithful. For technical engineering aspects this is even often advantageous because, in particular for a planar connection of surfaces of two objects, an accurate knowledge of the elevated regions thereof is often more important than the accurate determination of narrow recessed surface regions.
On the other hand, the resolution of tactile measurements, in particular for a more accurate measurement of surface depressions from the method-inherent limitations mentioned above, is no longer sufficient for many new applications.
Therefore, in the prior art approaches for contactless measurement, in particular by means of optical sensors, have been pursued in the meantime. By means of an optical sensor with an emitted measurement light beam, in particular from a laser, even surface depressions can be measured very accurately, as long as the focus of the measurement light beam, to be compared with the measuring tip of a tactile sensor, on the object surface is not larger than the structure of the depressions thereof. The resolution of optical measuring methods can accordingly be significantly higher than that of tactile measuring methods for an accurate measurement of surface profiles, in particular of the depressions thereof. Accordingly, a profile created by means of an optical sensor differs from a profile of one and the same object surface created by means of a tactile sensor. However, even a surface profile created by means of an optical sensor, in the same way as a surface profile created by means of a tactile sensor, constitutes an imaging of the actual object surface filtered in terms of its resolution on the basis of the physical dimensions of the “measuring tip”, wherein the dimensions of the optical “measuring tip”, in comparison with the measuring tip of a tactile sensor, can be regarded as converging towards zero or are negligible. Therefore, optical sensors and measuring methods for a coordinate measuring machine are suitable, in principle, for providing an actually object-faithful measurement of a surface profile.
Optical sensors that have been introduced into metrology with coordinate measuring machines in the meantime are based for example on laser light being radiated onto an object surface for interferometric measurements (EP 2 037 214). Methods based on white light interferometry (DE 10 2005 061 464) and chromatic-confocal methods (FR 273 8343) have also been proposed.
Optical sensors and measuring methods for a coordinate measuring machine are associated with a series of advantages: the measurement is carried out contactlessly, and the optical sensor can be led over an object surface more rapidly than a tactile sensor, with a smaller physical dimensioning of the “measuring tip”, as result of which a higher lateral resolution of the measurement is made possible.
Nevertheless, not only surface profiles created by means of tactile sensors but also those created by means of optical sensors always also include features which do not originate from the measured surface, but rather are caused by the measuring method. By way of example, DE 197 35 975 discloses measurement errors when determining the height of a surface on account of vibrations of the coordinate measuring machine used and method measures for suppressing these effects.
The measurement results of optical sensors, in particular for interferometric measuring methods, are often disadvantageously influenced by phase noise or speckle effects. Depending on the roughness of the object surface, for example, the phase of the light reflected from a surface can be altered in such a way that a distance measured to a targeted object point is incorrect. As a consequence of such local optical disturbance influences, surface profiles measured by means of optical sensors often have measurement errors, such as, for example, virtual singular peaks or depressions which, however, do not exist in the object surface.
For removing the data of such incorrect measurements from the measurement results, a post-processing of the measurement results is usually carried out by means of a suitable algorithm for filtering the raw data. An extremely simple data filtering is based on a “moving averaging” of the measurement results, which involves, proceeding from a first measurement value, averaging the measurement values of a predetermined number of laterally sequentially recorded measurement values with the first measurement values and assigning the average value determined to a measurement center point assigned to the relevant measurement locations on the measured object, wherein this averaging method is then continued progressively for all measured measurement locations on the object. Other known filtering techniques are based on triangular or polynomial kernel filters or on specifically defined transformation functions in the frequency domain.
Since measurements by means of tactile and optical sensors are subject to different disturbance influences, the filter algorithms have to be adapted to the respective type of sensor.
Filters used for processing the results of tactile measurements usually have a less intense effect since the physical dimensioning of the measuring tip of a tactile sensor already leads to a smoothing effect. When the filters are applied to the measurement data in the sphere center domain, in which measurement data smoothed by the measuring tip (compared with the actual surface profile) are already present, only a small loss of information takes place for the filtered data inverse-transformed into the object profile domain. Particularly the measurement values of critical elevations or peak heights of the surface are scarcely influenced by the data filtering in the sphere center domain.
Filters used for processing the measurement data of optical sensors are provided, in particular, for eliminating measurement errors caused by optical noise and speckle effects and often have a more intense effect. By way of example, firstly a prefiltering is carried out by means of a narrow rectangular filter, in particular for eliminating speckle effects. When choosing the filter width, the diameter of the generated light spot of the measurement beam emitted by the optical sensor on the object surface and “blurring” of the measurement data brought about by the scanning movement of the measurement beam over the object surface are usually taken into account. In a second processing step, an additional filtering is then often carried out by means of a triangular filter kernel. Particularly when relatively wide triangular filter kernels are used, the maximum height of very narrow or pointed surface elevations can truncated. On the other hand, when relatively narrow filters are used, optical noise and/or effects of surface roughnesses not relevant to technical applications are not adequately suppressed.
Since optical sensors supply measurement values directly in the object profile domain, the data are reduced during filtering, without an inverse transformation, as early as in the object profile domain, which can lead to a loss of information including about actual, and not only artificial, surface features.
What is disadvantageous about the prior art, furthermore, is that the known methods for processing measurement data of optical sensors of coordinate measuring machines often lead to a loss of information about significant structure details, in particular narrow elevations, of an object surface.
One problem addressed by the invention is that of providing an improved coordinate measuring method and an improved coordinate measuring machine, comprising an optical sensor, wherein measurement data detected by means of the optical sensor can be processed further at least indirectly with a processing functionality provided for tactile measurement data.
A further problem addressed by the invention is that of providing an improved coordinate measuring method and an improved coordinate measuring machine whereby it becomes possible for a surface profile of an object to be created more object-faithfully by means of optical sensors and a comparability with measurement data from tactile surface measurements of the same surface is provided. In this case, one specific problem addressed is to avoid losses of information about narrow surface peaks.